Does fspy add-on work in blender 2.81?
You installed the wrong file; you need to install fSpy-Blender-1.0.3.zip
 (or newer) from the Releases page.

The file you're using, fspy-Blender-master.zip is likely the source code and doesn't work as addon.

It's not very clearly stated on the page and it's easy to download the wrong file(I had the same issue :)